Ireland’s reputation as the “Emerald Isle” is linked to its climate, characterized by frequent precipitation that maintains the country’s vibrant green landscape. Situated in the North Atlantic, the island is in the path of weather systems bringing moisture-laden air across the ocean. This maritime influence results in a temperate oceanic climate (Köppen Cfb), meaning it experiences mild, humid, and changeable weather. Official data reveals that precipitation volume is not uniform, but varies significantly across the country.
The National Average Annual Rainfall
Based on the official climate averages from Met Éireann, the national annual average rainfall for Ireland (1991 to 2020 period) is approximately 1,288 millimeters (50.7 inches). This figure represents the average total precipitation recorded across the entire country. This national figure serves as a definitive benchmark, but it is a statistical average that smooths out significant regional differences. The totals are collected and analyzed over a standard 30-year period, which is the internationally recognized length for defining climate averages.
Geographic Variation Across Ireland
The total amount of precipitation is not evenly distributed, with a stark contrast existing between the eastern and western halves of the island. The east and southeast regions, particularly around Dublin, are the driest areas, typically receiving annual totals between 750 and 1,000 millimeters. Conversely, the western regions and upland areas experience substantially greater volumes of rain.
Rainfall in the west generally averages between 1,000 and 1,400 millimeters annually, demonstrating the Atlantic Ocean’s profound influence. This disparity is driven by the prevailing southwesterly winds, which carry moisture-rich air directly from the Atlantic. This air mass first encounters the high ground and mountainous areas of the west coast.
As the moist air is forced to rise over these hills and mountains, it cools, leading to the condensation of water vapor and the creation of precipitation. This process, known as orographic rainfall, causes mountainous districts in the west to record the highest annual totals, often exceeding 2,000 millimeters per year. For example, some weather stations in the high ground of counties like Galway and Kerry have recorded annual averages of over 2,800 millimeters. The eastern side of the country lies in the rain shadow of the western hills, explaining its lower precipitation totals.
Seasonal Rainfall Patterns
While rain occurs year-round, precipitation volume exhibits a noticeable seasonal trend, with the cooler months recording the highest totals. Winter, specifically the December-to-February period, is generally the wettest season across Ireland. December is typically the month with the highest average accumulation, recording approximately 142 millimeters of rain.
The period spanning late autumn into mid-winter also sees high average rainfall. The months of spring and early summer are comparatively drier, with April and May typically receiving the lowest monthly totals. Met Éireann data shows May is the driest month with an average of 79 millimeters, closely followed by April at 82 millimeters.
Even during the summer months, precipitation is a regular occurrence, but the totals are generally lower than in winter. August often emerges as the wettest of the summer months, with an average of around 103 millimeters, largely due to the increased frequency of summer frontal systems.
Understanding Rain Frequency Versus Intensity
The popular perception that it is always raining in Ireland is often a consequence of the high frequency of precipitation, rather than its intensity. Ireland’s weather is characterized by a high number of “wet days,” which are officially defined as any day that records 1.0 millimeter or more of rainfall. The number of these wet days varies geographically, ranging from about 151 days a year along the east and southeast coasts to around 225 days a year in parts of the west.
This high frequency means that measurable rain falls on a significant portion of the year. However, the nature of this precipitation is often light, taking the form of drizzle or light showers rather than heavy, prolonged downpours. The mild, humid maritime air rarely supports the kind of intense convective thunderstorms common in continental climates, which deliver a high volume of rain in a short period. This prevalence of frequent, low-intensity rain contributes substantially to the overall feeling of wetness.